In general, semiconductor memories may be divided into two types, volatile memories and nonvolatile memories. Volatile memories lose stored data when power is removed, whereas nonvolatile memories retain stored data when power is removed. Static random access memories (SRAMs) and dynamic random access memories (DRAMs) are two types of volatile memories. A SRAM cell is comprised of a bistable flip-flop, and data is stored by setting the state of the bistable flip-flop. In a DRAM cell, a capacitor is used to store data.
There are several types of nonvolatile memories. One type of nonvolatile memory is an Erasable Programmable Read Only Memory (EPROM). EPROMs can be erased and reprogrammed by the user using special ultraviolet light equipment. An Electrically Erasable Programmable Read Only Memory (EEPROM) is a type of nonvolatile memory that is electrically reprogrammable using a relatively high voltage, and does not require the special equipment needed for reprogramming EPROMs. MNOS (Metal-Nitride-Oxide-Semiconductor) transistors, SNOS (Polysilicon-Nitride-Oxide-Silicon) transistors, SONOS (Polysilicon-Oxide-Nitride-Oxide-Silicon) transistors are also used for nonvolatile storage, where the storage of charge occurs in the nitride layer.
Nonvolatile RAMs, or NVRAMs, are memories which combine the high speed of SRAMs and non-volatility of a nonvolatile memory, such as an EEPROM. The memory cell of a NVRAM includes a SRAM portion and a nonvolatile portion. During normal operation, the NVRAM appears like a SRAM to a user. However, upon power interruption or any other interrupt sequence, data is transferred from the SRAM portion to the nonvolatile portion. The nonvolatile portion retains the data for an extended period of time. The nonvolatile portion of the NVRAM can include any of the above nonvolatile cells, or others not listed, that have two stable threshold voltage (V.sub.T) states. The state of the device can be changed from one stable state to another by altering the charge stored on either a floating silicon gate as in a floating gate transistor of an EEPROM, or in a gate dielectric film as in the MNOS device.
A problem common to many of the above nonvolatile memories is that they require relatively large programming voltages of about 10 to 15 volts for changing their threshold voltages. Also, they can only be re-programmed a limited number of times (typically about 100,000 cycles) before they are unserviceable. In addition, exposure to the large programming voltage can have a detrimental effect on the SRAM portion of the NVRAM, complicating the design of the NVRAM.
Another problem with current NVRAMs is the large size of the cell. A large cell increases the size and cost of the NVRAM array. The large array is due, at least in part, to the increased spacing between elements because of the high programming voltage.